1. Field of the Invention
The present invention relates to high specific speed fan propellers in general and more particularly to commercial direct drive fan propellers used for ventilation.
2. Description of the Related Art
In a fan which is used to move air, it is important that there be generally uniform pressure and velocity across all areas of the propeller disc. Output power is the product of volume flow through the fan orifice and the increase in total pressure. Uniform pressure minimizes kinetic energy losses. More particularly, if the pressure change is less at one area of the propeller disk, for example, near the hub, there may be backflow at that area.
The pressure developed depends on the square of the velocity. The velocity of a blade is much greater at the tip than at the root due to the difference in diameter. In the past, many commercial fans were belt driven which enabled the speed of the blade to be easily controlled. However, the additional parts necessary for a belt driven fan are associated with problems such as belts breaking and tension problems. Therefore, a direct drive fan is desirable. With a direct drive fan, it is difficult to control the speed. The speed of an electric motor for 60 cycle current is equal to 7,200 divided by the number of poles. The load on the motor will slightly decrease this synchronous speed. With an eight pole motor, the approximate speed, taking into account speed reduction due to the fan load, is approximately 800 RPM. This speed is significantly greater than the previous belt driven speeds.
By increasing the speed of the blade, the tip area of the blade must be decreased. To keep sufficient pressure near the hub, it would be desirable to have the blade be much wider near the hub. However, the resulting blade shape is not practical, as difficulties in twisting the blade result.
In the field of airplane propellers, it has been proposed to supplement the blade area near the hub by adding auxiliary blades. Although the appearance of an airplane propeller may be somewhat similar to that of a high specific speed fan propeller, the two are very different. Airplane propellers are designed to operate at a high air velocity through the disk, around 15,000 feet per minute. This higher velocity allows the angle which the blades make with the disk of rotation to be greater than that of fan blades. For example, the angle an airplane blade makes with the disk of rotation is approximately 25.degree..
Moreover, the function of a airplane propeller is very different from that of a commercial high specific speed fan. The airplane propeller's function is to develop thrust, not pressure. The output power of an airplane propeller is the product of a propeller's thrust and the airplane velocity. The propeller's thrust is the sum of the pressure differential of each blade increment from the blade hub to the blade tip. Further, the function of the airplane propeller near the hub is not critical. In an airplane propeller there is no need for uniform pressure addition. If the pressure addition is less near the hub, the performance of the propeller is not significantly impaired. Because uniform pressure addition is not critical, it is possible to make a useful airplane propeller with a simple blade root. In a simple blade root, the function of the blade root is mainly to support the rest of the blade, not to add pressure. Airplane propellers are generally forged and have a thick blade root with poor aerodynamic qualities. In a fan propeller, the aerodynamic qualities of the blade near the hub need to be quite good. The fan blade is often made of sheet metal having uniform thickness so that the blade functions near the root in a manner similar to how it functions near the tip.